Conventional ion imaging techniques utilized grid electrodes to extract and to accelerate ions toward the detector. The disadvantages of grid electrodes caused transmission reduction, severe image distortions and image blur due to the non-point source geometry. All these problems can be solved by the utilization of an open lens electrode assembly. In velocity mapping, the extracting electric field of an open electrostatic lens that projects the ion cloud onto the detector. The major advantage of the combination of ion lens optics and two-dimensional detection is that ions from different positions with the same initial velocity vector would be mapped onto the same position on the detector, which was named ¡§ velocity map imaging .¡¨ The kinetic energy resolutions achievable with this method are not generally considered as being competitive with the best photofragment translational spectroscopy technique. But Ashfold and co-workers have demonstrated that velocity imaging methods can provide dissociation energy with one wavenumber resolution, i.e., it compares favourably with all rival photofragment translational spectroscopy techniques. We construct an ion imaging apparatus and the pressure inside can be maintained at ~ 10-6 Torr with differential pumping when the pulsed nozzle is shut off. The pressure in the source chamber raises from 2.1¡Ñ10-6 to 1.0¡Ñ10-5 Torr and the pressure in the photolysis chamber raises from 2.4¡Ñ10-7 to 3.6¡Ñ10-7 Torr, when the pulsed nozzle is turned on with a stagnation pressure at 3 bar. Because reactive chemicals attack the piezo disk translator and ruin the Viton O-ring, a modified pulsed nozzle and Teflon O-ring are adopted to overcome these problems. This pulsed nozzle is mounted on a three-dimensional translational stage such that the nozzle can be aligned inside vacuum. The homogeneity of the accelerating electric field is crucial to the performance of the ion imaging apparatus. To meet this requirement, parallel electrodes of identical dimensions have been assured in the manufacture of the ion lens assembly.